It is extremely desirable in an optical CD writer as well as other CD systems to place the laser electronics, generally the laser driver and HFI (High Frequency Injection) circuitry, on the optical head with the laser. This is true because transmission line effects become less critical for laser driver rise/fall times at relatively small distances from the laser, EMI from HFI signals are easier to contain from the rest of the system not associated with the head, and overall the signal-to-noise ratio is improved. On the other hand, this close coupling of the high current laser driver circuitry, with a high frequency injection circuit, for quieting the laser noise in a small package or module, creates a system's packaging paradigm of trying to manage rather stringent thermal and electrical coupling requirements.
The integrated laser module of the present invention is designed to optimize rise and fall times in the order of 1-2 ns (nano sec), deliver up to 400 ma (milli-amps) of combined laser drive current during a Write mode, and to minimize EMI from a 200-500 Meg Hz high frequency injection (HFI) circuit in a cost effective, light weight, small package. Since lasers typically have parameters which shift adversely as a function of temperature, it is also important to provide an extremely low thermal resistance path for the laser driver which, in the preferred embodiment of this invention, is a custom ASIC circuit. For example 400 ma of Write current can result in up to 1.6 watts of power being generated during a Write cycle which typically exists for a 50% duty cycle at 5-6 Meg Hz. These temperature dependent laser parameters include wavelength, threshold, slope efficiency, and leakage, the change of which can significantly impact mark formation during a Write mode in an optical CD system. The threshold current change and slope efficiency changes at higher temperatures require that the laser driver current be increased to maintain a constant power level out of the laser. A constant power level is required for consistent writing quality and is critical as writing speeds approach and exceed six times the read speed. This increased current further adds to the temperature rise of the junction of the laser which in turn requires more current to the laser in order to maintain constant power. The present invention addresses this problem.